Here is a basic explanation.
As a general rule when you have a "Manufacturer's Coupon", which is what most grocery coupons are, you are paying the full price and are receiving a rebate from the manufacturer. Therefore you pay tax on the purchase. The store is acting as agent for the manufacturer, and giving you the rebate on the spot.
The two exceptions to this are both when the store itself is giving you a discount, which is a reduction of the selling price.
An example might be you go to a restaurant and there is a promotional coupon from the store for a certain amount (or percentage) off the meal, such as buy one and get a second for half price. This is an unrestricted discount and you pay the tax on the final billed price. If you have the Tables in Wonderland Card, and use it for a discount on meals at Disney, you will pay the gratuity at the full price but the sales tax only on the 80% of the menu.
If you have a store that doubles coupons, and let's say you have a 50¢ manufacturer's coupon for a $3.00 taxable item you will get a discount of $1.00 but you will pay the tax on $2.50. This is because the store is giving you an unrestricted discount of 50¢ which lowers the selling price to $2.50 and you are receiving a 50¢ rebate from the manufacturer.
This is the simple explanation, and is followed by all states that have sales taxes.
Mike (CPA Retired)